Linear motors have been used for many years as positioning devices. For many applications, it is important that the linear motor accomplish its positioning function at a predetermined velocity, or within predetermined velocity limits. An example of such an application is in positioning of a read/write head of a magnetic disk memory system, the head having to approach a rotating disk face at a velocity lying between predetermined velocity limits. Once the head is located adjacent to the disk surface, velocity information can be read by the head from the disk surface and utilized to control head velocity across the disk surface. However, if the head is not adjacent to the disk surface, velocity information must be obtained by another means. Conventional systems utilize an external velocity transducer which is connected either to a voice coil of the linear motor, or to a transducer-containing structure which is moving in proportion to the velocity of the voice coil. The velocity transducer provides a feedback signal to the voice coil driver thereby maintaining the velocity of the voice coil within predetermined limits.
Conventional devices to provide a voltage proportional to velocity of a linear motor voice coil typically consist of a separate electromagnetic type velocity transducer mounted in some way so that it attaches to a structure which moves in proportion to movement of the voice coil. The stationary part of the velocity transducer is typically fixed with respect to the structure to which the linear motor is mounted. Disadvantages of a separate velocity transducer include both cost and mechanical complexicies. A separate transducer is relatively expensive with respect to the total cost of a linear motor. In addition, mechanical resonances within the transducer itself and mass added to the structure being moved by the linear motor are both undesirable side effects of a separate velocity transducer, and sometimes mask the actual velocity of the voice coil. These resonances sometimes move oppositely with respect to the actual movement of the voice coil thereby resulting in a velocity indication from the velocity transducer which is not representative of the actual movement of the voice coil itself. Although disadvantages associated with conventional methods of determining velocity can be largely eliminated by presently known techniques, all of them increase the weight to be moved by the linear motor voice coil and result in increased cost. The present invention discloses an apparatus whereby velocity of the linear motor voice coil over a predetermined portion of its travel can be determined without use of a velocity transducer physically separate from the voice coil or the addition of a significant weight to the mechanism being moved by the voice coil.